Recently, heat-shrinkable plastic films have widely been used aiming at wrapping of packaged articles for improvement in appearance, at packaging for avoiding direct impact of contents, and at label package serving as protection of glass bottles or plastic bottles and display of articles. In plastic materials used for these purposes, are drawn films, such as polyvinyl chloride films, polystyrene films, and the films are used for labels, cap seals, or integrated packaging, for various containers, such as polyethyleneterephthalate (PET) containers, polyethylene containers, and glassware.
However, on one hand, polyvinyl chloride films have outstanding shrink characteristics, but have low heat-resisting property. Furthermore, it has problems of generation of hydrogen chloride gas or dioxins in incineration. In addition, use as shrink labels of heat-shrinkable vinyl chloride resin films for, such as PET containers, causes a problem of needing separation of the labels from the containers, in recycling use of the containers.
On the other hand, a polystyrene film can be evaluated in terms of good finishing appearance after shrinkage; however it is inferior in solvent resistance and therefore requires use of an ink with a special composition at the time of printing. Further, recently, a heat shrinkable film has been employed for PET bottle labels for hot beverages; however with respect to a heat shrinkable polystyrene film, in the case of storage in warming equipments such as hot warmers, there is a problem that the shrinkage label is at once melted when being brought into contact with a high temperature hot wire or the like. Further, a polystyrene resin has to be incinerated at a high temperature and has a problem that a large quantity of black smoke and malodor are generated at the time of incineration.
A polyester film free from these problems (excellent solvent resistance, excellent heat resistance, and excellent environmental aptitude) has been used as a shrinkable film alternative to a polyvinylchloride film and a polystyrene film.
In production of labels etc., following methods are usually adopted. That is, raw material polyester is continuously extruded in a molten state. An undrawn film is manufactured. Subsequently, the undrawn film is drawn and wound to give a heat-shrinkable polyester film roll. The film in the roll is then unwound and slit into a film with a desired width, the slit film being rewound into another roll. Subsequently, various information and figures such as product name etc. are printed on the resulting film. After printing, the film is folded and bonded along both edges by means of, for example, solvent adhesion, to give a tubular film (in tubing process). Meanwhile, there may be cases where the printing and the slitting processes are performed in a reversed sequence. The tubular film thus obtained may be cut into tubular labels with a desired length, being further converted into bags by bonding along an edge of openings thereof (for example, patent document 1).
Subsequently, containers wrapped in the labels or the bags above are passed, for example on a belt conveyer, through a shrinking tunnel (a steam tunnel) wherein heated steam is blown in for heat shrinking or a hot-air tunnel wherein hot air is blown in, to give final products (labeled containers) having the labels or the bags tightly attached thereon.
One of important properties which are required as a film to be used for container's labels is transparency. As described above, in many cases, letter information and designs of various kinds of product names or the like are printed on the film to be used for container's labels and in the case of a transparent label, printing is done in the side of the film which is contact with the container and if the transparency is low, it becomes impossible to print clear printing patterns. Moreover, in unprinted portions, there is a problem that it is difficult to confirm the color and packed amount of the content such as a beverage in the transparent container.
Furthermore, one of characteristics of a polyester film among shrinkable label materials is excellent gloss, and if the gloss is low, it may lead to an adverse effect on commercial product images, for example, an effect to diminish high grade impression of coated commercial products.
As a method for processing a shrinkable film, there is a case that a metal vapor deposition process is carried out on a label to provide metallic appearance. The process is often carried out in the side which is contact with a container, and in the case of a film with inferior transparency and gloss described above, it is difficult to give appearance of the covered container surface to be like a metal surface.
As described above, improvement of transparency and gloss for a film for shrinkable labels is important for expectation of an effect to improve the designs and commercial product images. The invention aims to further improve transparency and gloss of an existing heat shrinkable polyester film.
Further, since a conventionally known polyester film is an insulator, the film has a problem that static electricity is easy to be generated and accumulated. For example, static electricity causes winding of the film on a roll and electric shock on a human body in the film production process, printing on the film, or bonding films to each other, and thus it makes handling of the film complicated. Further, static electricity becomes a cause of so-called printing beard and stains on the film surface and may possibly lower the commercial product value. Consequently, it is desired to provide a polyester film with suppressed static electricity generation and accumulation.
Patent document 1: Japanese Patent Laid-Open No. 2005-335111